1) Field of the Invention
The invention relates to a method for optical pumping of a set of particles such as atoms, ions, molecules or other particles, with which formation of black states may be avoided. It also relates to a device for applying this method.
The invention finds interesting applications in atomic resonators and generally in all the instruments which use a source of particles and at least one laser beam capable of performing optical pumping of these particles. A particularly representative application is the atomic clock with an optically pumped cesium jet, the principle of operation of which is described later on. However, this example would not be able to limit the scope of the invention.
2) Description of Related Art
A particle may be characterized from the energy point of view, by discrete levels with a variable lifetime. FIG. 1 in a simplified way, schematically shows a representation of a particle having a level A, a level B and a level E. The lifetimes of levels A and B are long relatively to the time for optical pumping, a notion to be explained in the following, which itself is long relatively to the lifetime of level E.
As illustrated by FIG. 2, in the case of a cesium atom 133Cs, level A corresponds to the hyperfine fundamental state (62S1/2, F=3), level B corresponds to the hyperfine fundamental state (62S1/2, F=4) and level E for example corresponds to the hyperfine excited state (62P3/2, F=4). Further, in this case, each level is degenerate into sub-levels (also called Zeeman sub-levels), respectively designated by (62S1/2; F=3; mF=−3, −2, . . . , +2, +3) and (62S1/2; F=4; mF=−4, −3, . . . , +3, +4) for both fundamental levels, and (62P3/2; F=4; mF=−4, −3, . . . , +3, +4) for the excited level.
By optical pumping is meant the method by which a fraction or the totality of the particles filling a level A (also called the population) may be transferred to a level B, or vice versa, by means of an interaction between light and the particles. The optical pumping time is defined as the time required for transferring by this method, particles from A to B, or vice versa. In the case of the 133Cs atom, the hyperfine optical pumping method is further distinguished from Zeeman optical pumping. Hyperfine optical pumping depopulates one of the fundamental levels (62S1/2, F=3) or (62S1/2; F=4) to the benefit of the other one by an optical interaction via the excited state (62P3/2, F=4) for example, no distinction being made between the Zeeman sub-levels. Zeeman optical pumping depopulates certain Zeeman sub-levels to the benefit of a single sub-level or a superposition of Zeeman sub-levels of a fundamental hyperfine level, by one or more optical interactions via one or more excited states.
Experimentally, in the case of the 133Cs atom, hyperfine optical pumping is achieved by illuminating a collection a particles (as a jet or a cell) with an optical beam emitted by a discharge lamp or a laser, with a frequency corresponding to one or more allowed optical transitions between a fundamental level (62S1/2; F=3) or (62S1/2, F=4) and one more excited levels, for example (62P3/2, F=4). With polarized light emitted by a laser, coherences may appear between the Zeeman sub-levels of the fundamental level coupled to the laser, giving rise to so-called “black” states, i.e., transparent to the hyperfine optical pumping process. Dimarcq et al. (IEEE Transactions on Instrumentation and Measurement, 42 (2), April 1993, 115-120) have demonstrated on an experiment with an atomic cesium jet pumped by a laser, that the noise added by the residual fraction of atoms trapped in the black states may considerably reduce the atomic signal-to-noise ratio and therefore become detrimental to the performance of an atomic resonator.
The basic structure of an atomic clock with an optically pumped cesium jet is illustrated by FIG. 3. A source 10 positioned inside a high vacuum chamber 12, produces a fast or slow atomic cesium jet 14. A fast jet is for example generated by an oven, whereas a slow jet is produced by a source of atoms slowed down or cooled by laser. A first so-called ‘preparation’ laser beam 16, crossing the atomic jet 14 at the outlet of the source 10, achieves the inversion of population required between both fundamental hyperfine states by a hyperfine or Zeeman optical pumping process. The atoms of the jet 14 then undergo a transition between both fundamental hyperfine states in a resonant cavity 18 injected with a microwave from a local oscillator 20. A second so-called ‘query’ laser beam 22, crossing the atomic jet 14 at the outlet of the resonant cavity 18 detects by a hyperfine or Zeeman optical pumping process, the atoms which have constructively undergone the microwave transition. A detector 24, located facing the query area of the atomic jet 14, collects the fluorescence light re-emitted by the atoms having undergone the optical pumping processes. The intensity of this light signal gives information on the frequency tuning between the microwave injected into the resonant cavity and the atomic transition between both fundamental levels. The signal from the detector 24 further provides servo-control of the microwave frequency of the local oscillator 20, by an adequate system. Both preparation 16 and query 22 optical beams may stem from a single source or multiple sources. They may further result from a superposition of several optical beams with different frequencies.
As mentioned earlier, the signal-to-noise ratio of the fluorescence light in the query area, which conditions the frequency stability performance of the atomic resonator, is degraded by the presence of atoms trapped in black states. Several solutions for suppressing them already exist but they all have a certain number of drawbacks.
A first possibility described in an article of Giordano et al. (IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 38 (4), July 1991, 350-357) consists of establishing an intense static magnetic field (larger than 300 mG) with an adequate orientation, i.e., perpendicular to the axis of polarization in the case of linear polarization of laser beams (16, 22) in both regions of hyperfine optical pumping. This has the effect of redistributing the Zeeman populations inside each fundamental hyperfine level, and therefore destroying the black states. This solution, although effective, proves to be difficult and complex to apply because of the requirement of several contiguous magnetic field areas with very different intensities (ten times weaker between the pumping areas than in the latter).
A second solution, described in an article of Dimarcq et al. (Journal of Applied Physics 69 (3), February 1991, 1158-1162), consists of selecting an optical pumping transition which does not create any black states such as for example the 3−4π (D1) transition at 894 nm for the 133Cs atom. Unfortunately, the selection of such a transition strongly reduces the degrees of freedom for optimizing other parameters (hyperfine optical pumping efficiency and fluorescence rate). Further, the availability of performing laser sources at these wavelengths is not guaranteed.
A third solution, described in the article of Shirley et al. (Proceedings of the 1994 IEEE Precision on Electromagnetic Measurements, 150-151), consists of perturbing the polarization of the laser beam, at the origin of the trapping of the atoms in the ‘black states’. As illustrated by FIG. 4, in which the preparation area is greatly enlarged, this perturbation is produced by creating an optical stationary wave having a polarization gradient substantially parallel to the direction of propagation of the laser beam 16. The stationary wave is produced by retro-reflecting the linearly polarized laser beam 16, by means of a plane mirror 26 located on the path of the laser beam 16, downstream from the pumping area. The longitudinal periodical polarization gradient 28 is achieved by inserting a quarter-wave plate 30 between the plane mirror 26 and the atomic jet 14. In such a stationary wave, polarization changes significantly every quarter wavelength of optical wavelength (0.2 μm for 133Cs). This solution, certainly elegant, nevertheless has the drawback of introducing two additional optical components with respect to the standard configuration. Further, it is not totally effective if the polarization gradient is perfectly perpendicular to the atomic jet, which is the case in the standard configuration of the atomic clock. In this case, the atoms may cross the laser beam without being subject to any significant change in polarization of the light, and the black states are not destroyed, or are only incompletely destroyed.